1. Field of the Invention
This invention relates to vertical lift aircraft rotor system maintenance, specifically to the identification of rotor faults and dynamic instability in said systems using optical rotor blade tracking data.
2. Prior Art
Vertical lift aircraft rotor systems, rotor head and attached blades, deliver significant vibration into the aircraft. Asymmetric rotor weight, variations in lift and drag among the plurality of rotor blades, chord wise imbalance, general system ware and tear, mechanical faults, and other factors all contribute to induce vibration. The same factors that induce vibration can also threaten the safety of aircraft and crew. Over the years a variety of inventions have received patents with the primary focus of reducing the vibrations. A number of these focus on two primary sensing devices. These devices are accelerometer vibration sensors and optical or electronic blade tracking devices. The devices have been used alone and in combination by various organizations to reduce vibration in vertical lift aircraft. Optical trackers are typically used to view the aircraft rotor blades in flight and reduce the variations each in track. Track being the flight path that each blade takes in its rotation. Variations in track are indicators of variations in lift and can be major contributors to vibration. Accelerometers are typically attached to aircraft structure and sense vibration at the point of attachment. Accelerometer locations are selected for the sensitivity to vibration frequencies of interest, generally the primary rotor rotation frequency and its harmonics. The vibration levels at the locations selected are sense the levels of rotor induced vibration. When the sensed vibration is above established limits the rotor head and or attached blades are adjusted to lower vibration to acceptable levels. Supplementary sensors are included to detect rotor rotational speed. Signal processors or computers are used to analyze the complex signals collected by the various sensing devices and provide system adjustment recommendations.
Accelerometer only devices such as U.S. Pat. No. 3,938,762, U.S. Pat. No. 4,937,758, U.S. Pat. No. 6,415,206 and U.S. Pat. No. 6,574,572 rely on a plurality of accelerometers distributed about the aircraft. Data received from said accelerometers are processed using a computer algorithm. The result of said algorithm is to be an optimized set of adjustment of the rotor and its plurality of blades. The intended result is to reduce aircraft vibration. Accelerometers easily detect the aircraft's primary rotational frequency and several harmonics of that frequency. However, using accelerometers to detect rotor or blade component faults and dynamic instabilities is difficult if not impossible. Rotor induced vibrations enter the aircraft through two primary paths. Path one is through direct connection to the aircraft structure. Path two is air loads flowing from the turning rotors. Path two is pressure waves and turbulence impinging on the aircraft structure. The loss of detailed mechanical vibratory data in path two driven vibrations is obvious. In path one, the vibration must pass from the rotor head then through several mechanical interfaces such as gear boxes, engine interfaces, a variety of damping systems, and structural connections before reaching the accelerometers. Significant fault information never reaches accelerometers due to high information path interference and noise.
Optical and electronic tracking devices such as those taught in U.S. Pat. No. RE33,097 (original U.S. Pat. No. 4,604,526); U.S. Pat. No. 4,887,087; U.S. Pat. No. 5,929,431; U.S. Pat. No. 5,249,470; U.S. Pat. No. 6,448,942 and commercial derivatives of these patents are non destructive testing devices. These devices will be generically referred to as the “Tracker” here after. These devices mark the passing of a rotor blade though the viewing port or sensor detector and record the vertical position in the devices field of view, the time passage of each of a plurality of blades for each revolution of the rotor, and the total time required for the rotor to make the current revolution. Computational units are included within said devices and/or external to said devices. The computational units typically average several revolutions of rotor data, revolution to revolution data, to calculate track and lead/lag parameters. This averaging is used to smooth the variations in height and time occasioned by the blade over the several revolutions of the rotor. The revolution to revolution data collected are normally discarded once they have been included into the averaging calculations. Discarding the revolution to revolution data is encouraged in order to minimize the need for large amounts of data storage. More to the point, the use of the data revolution to revolution data was thought be of little or no value.
Optical blade trackers were used prior to the 1980s. The early systems depended primarily upon operators to visually determine the track of the rotor blades, in effect “averaging by eye”. Optical blade trackers aided and controlled by computers to collect, analyze, and store data came into general use in the early 1980s. Today these devices are used extensively in helicopters, tilt rotor aircraft, and whirl towers. Whirl towers are helicopter rotor blade dynamic balancing facilities. Whirl towers are used to control the dynamic performance of rotor blades so that said blades can be interchanged within any of a set of blades of the same kind. Reconditioned and repaired rotor blades will also be dynamically tested at said towers to ensure that the blades meet the manufacturer's very tight weight and dynamic tolerances.
Through the years, mechanical and dynamic problems within rotor systems and associated blades have been encountered on rotor tower installations as well on various types of aircraft. The problems manifested themselves as anomalies within rotor tracking data collected from the aircraft or whirl tower. Although accelerometers were in use when many of the problems rotor system oriented problems were encountered, the accelerometer data did not materially aide in the diagnosis of these problems. Faults were often revealed in the affected aircraft or towers after physical investigations were made. The idea that optical tracking devices could be used to detect rotor head and blade faults has been considered by various organizations for years. Attempts by several companies to utilize optical tracking devices to analyze rotor faults have failed. A testament to this is the fact few, if any, commercial systems have a method of recoding and maintaining revolution to revolution data within their normal systems without evoking a specialty program used for engineering purposes. The engineering purposes noted here are generally to be a quality check of the tracking device.
This invention is one result from a body of work undertaken to improve rotor blade whirl tower, helicopter, and tilt rotor tracking techniques. Within the work effort it was decided to include the ability to collect and store revolution to revolution blade track data as a normal operator selectable function. The decision was based on the knowledge that the signs of failure and fault were resident in the data. The ability to extract fault data from said data would elevate the rotor tracking from a single to a multi-dimensional tracking device. Said ability would make the Tracker a viable maintenance tool for rapidly assessing the mechanical health of rotors and its associated blades. The question to be answered was how to extract the valuable information from the revolution to revolution data generated by the Tracker.
Rotor systems, including the plurality of attached blades, are designed to act as a unit for primary purpose of managing lift in a manner as to allow an aircraft to fly and be controlled directionally. The aerodynamic characteristics; airfoil cross sections, chord, blade twist, materials, weight distribution, etc.; of the rotor blades are fixed and very tightly controlled and held to tight tolerances throughout the manufacturing process. None-the-less, small variations will be found within the individual blades. Trim tabs, methods of adding or moving weight, and other techniques for modifying blades provided for the purpose of changing the lift and drag of an individual blade. Rotor heads are built with precision but some small variations are inherent in manufacturing.
Marrying a rotor head and a plurality of blades together on a specific model of vertical lift aircraft require that the flight patterns and dynamic responses of each blade on a rotor will be within very small tolerances from rotor blade to rotor blade, hub to hub, and from aircraft to aircraft. Normal wear and tear and occasional damage to blades and rotor demand adjustments of the blade's lift and drag qualities. Said adjustments may include altering pitch change link and/or trim tab settings, and adding or subtracting weight from the blade or rotor hub. Wear and tear to the blade, including field maintenance practices, can change a blade's weight distribution. These weight changes can radically change the flight qualities of a rotor blade by modifying the blades centrifugal pitching moment profile. Uncontrolled centrifugal moment changes may cause a blade to climb or dive excessively. The climbing and diving tendency can increase rapidly with addition of aircraft weight or flight loads and become so severe that loss of control of the aircraft can occur. Excessive wear and other faults can go unnoticed especially when tempo of operations are high, such as in military operations, search and rescue, fire fighting, logging, and other high energy situations where there is a demand for high vehicle availability. The need to rapidly access the rotor system and blade integrity in high tempo of operation situations is clear. The ability to detect faults is an equally powerful incentive when safety and efficiency of vertical lift operations are considered.
Significant analysis of revolution to revolution data from various aircraft and helicopter blade rotor tower installations were conducted. It was clear that a variety of unquantifiable aircraft and blade disturbing events such as pilot or automated control inputs, weather and air mass disturbances, load variations, and more were resident in the revolution to revolution data. These facts had been seen to lead others abandon using revolution to revolution data as a fault detection and maintenance tool. Our research took us to utilize statistical analysis and found that it was without promise. Something more unorthodox was required to use the subject data productively. Additional research and analysis led to a methodology of deconstruction of track, lead/lag, and rotor timing data. Said data are then recombined in way that provides the ability to recognize rotor related problems, rapidly access the dynamic health of a helicopter rotor system, and reduce track and balance evolutions to a small fraction of time generally required.